1. Field of the Invention
The present invention relates generally to a video projecting apparatus including a plurality of cathode ray tubes and a plurality of projecting lenses. More particularly, the present invention relates to a color video projecting apparatus which eliminates the distortion of projected images on a screen by optically correcting these images.
2. Description of the Prior Art
A typical color image projector includes three cathode ray tubes, each cathode ray tube providing an image of a respective primary color i.e. red, green or blue color. The color image projector projects primary color images from the respective cathode ray tubes onto a screen through respective projecting lenses and produces a color image on the screen by mixing and compounding three primary color images. The light axis of each of the cathode ray tubes of the color projector is arranged coaxially with the axis of the corresponding projecting lens. In addition, the light axes corresponding to the color images of the respective cathode ray tubes are converged on the screen at the center thereof. Accordingly, since the incident angles of the respective light axes on the surface of the screen are different from each other, the distorted shapes of the projected color images differ. As a result, when the compound image is formed, misalignment results and a good color image can not be obtained.
In order to remove the above distortion, electric correcting circuits have been provided in the deflecting systems of cathode ray tubes corresponding to the respective colors to predistort images on the cathode ray tubes to predetermined shapes to obtain a color image on the screen with no distortion.
This, however, requires separate correcting circuits in the respective cathode ray tubes, resulting in a complicated construction and adjustment of the respective color images.
In order to correct the above distortion, one system, described in U.S. Pat. No. 4,087,835 to Nishimura et al. may be utilized. This system is also described in U.S. Application, Ser. No. 903,002, by Ohmori and Ito, filed May 4, 1978 for "PROJECTING APPARATUS", now abandoned.
The arrangement disclosed in the above-identified U.S. application will be now described with reference to the drawings. FIGS. 1 and 2 are schematic diagrams showing the apparatus described in the above-mentioned U.S. application which provide a base for the present invention. Referring to FIGS. 1 and 2, the apparatus includes, there are provided a projecting lens 4RB and a half mirror 3 common to red and blue cathode ray tubes 1R and 1B, and a projecting lens 4G for a green cathode ray tube 1G, respectively. Projecting lenses 4RB and 4G are so arranged that their optical or light axes 5RB and 5G are parallel with each other and perpendicular to a screen 2. In this manner, the light axes 5RB and 5G are parallel to a normal 6 to the screen 2 at its center O.sub.s, and images 7RB and 7G, which are projected onto screen 2 by lenses 4RB and 4G, are free from any distortions, as shown in FIG. 3.
However, in utilizing this system, each of the light axes 5RB and 5G is displaced from the center O.sub.s of the screen 2 by a distance d, so that, as shown in FIG. 3, the images 7RB and 7G are shifted in the right and left directions by the distance d, respectively. Referring to FIG. 3, an image 7C is shown as a dotted line representing the image position when the light axes from the respective images are located at the center O.sub.S of screen 2, and one-dot chain lines 8RB, 8G and 8C show the center lines of the images 7RB, 7G and 7C, respectively. In order to correct this misalignment in the above apparatus, the cathode ray tubes 1R, 1G and 1B are outwardly and expaxially displaced by a predetermined length e with respect to the light axes 5RB and 5G in accordance with their distances from the center of the screen 2. Thus, without using any electrical correcting circuits, the images 7RB and 7G can be made coincident with each other at the center of the screen 2 and any misalignment is eliminated.
However, even with the above optical correction, there remains a distortion in the images 7RB and 7G due to other causes. For example, since the phosphor screens of the cathode ray tubes 1R, 1G and 1B are covered by glass face plates which each have a predetermined thickness, the image reproduced on each of the phosphor screens appears to be in a floating state near the surface of the face plate due to the refraction of glass when viewed from the outside of the face plate, and it is this floating image caused by refraction of the glass which is projected onto the screen 2.
When the cathode ray tubes 1R, 1G and 1B are displaced from the light axes 5RB and 5G, as set forth above, the apparent thicknesses of the face plates of the cathode ray tubes 1R, 1G and 1B are different at positions near and away from the light axes 5RB and 5G. Referring to FIG. 4, in the instance where an image 12 is reproduced on a phosphor screen 11 of the cathode ray tube 1R, the angle between the line from the end of the image 12 near the light axis 5RB to the center of the lens 4RB, and the surface of the phosphor screen 11, is .theta..sub.1, while the same angle at the other end of the image 12, away from the light axis 5RB, is .theta..sub.2, .theta..sub.1 being larger than .theta..sub.2 (.theta..sub.1 &gt;.theta..sub.2). As a result, apparent thicknesses l.sub.1 and l.sub.2 of a face plate 13 of the cathode ray tube 1R, thicknesses l.sub.1 and l.sub.2 being measured at opposite ends of the image 12 viewed from the lens 4RB, can be expressed as follows, if the true thickness of the face plate 13 is given as l ##EQU1## hence EQU l.sub.1 &lt;l.sub.2
If the apparent thickness of the face plate 13 is different at the respective positions, as set forth above, the apparent location of the image 12, which is caused by the refraction of the glass of the face plate 13, becomes varied due to the apparent thicknesses of the face plate 13 at the respective positions. In this manner, the image to be projected becomes inclined as shown by a dotted line 14 in FIG. 4, i.e., the image 14 to be projected is not parallel to the screen 2. As a result, although the light axis 5RB of the lens 4RB is perpendicular to the screen 2, the image projected onto the screen 2 is a distorted trapezoid, shown in FIG. 5 by a solid line.
Further, the inclinations of the images 14 of cathode ray tubes are opposite to each other as shown in FIG. 4, resulting in the image 7G projected from the cathode ray tube 1G being an inverse trapezoid to the image 7RB from the cathode ray tube 1R, as shown in FIG. 5 by the dotted line. In this manner the image 7RB does not coincide with the image 7G entirely, resulting in a misalignment in the reproduced image on the screen 2. It is to be noted that the image projected from the cathode ray tube 1B is coincident with the image 7RB from the cathode ray tube 1R since the optical path of both the cathode ray tubes 1R and 1B are same.